In oil well pumping, a string of sucker rods, or occasionally a long flexible pumping strand, or cable, which extends down the oil well tubing within the well casing is used to stroke a pump at the bottom of the oil well. On the upstroke, fluid in the well is lifted and the basic load consists of the weight of the string of pumping rods plus the weight of the fluid in the well. Dynamic loads are also imposed on the rod string and fluids by the motion of the rod string.
While the component sucker rods or pumping strand are formed from high strength steel they have a certain elasticity and in a deep well the cumulative elasticity of the lengths of sucker rod provides an elastic member with a very significant amount of stretch. The stretch of a very long pumping string may in fact be so great that a short stroke pump at the bottom of the well will not even be operated, all of the movement imparted to the pumping string by a short stroke pumping apparatus at the surface being expanded in stretching the pumping string, leaving insufficient movement at the bottom of the well to operate the pump. For this and other reasons, so-called long stroke pumping units have become more and more popular in recent years. The longer stroke imparted to the rod string by the long stroke pumping unit is sufficient to operate a pump at the bottom of a deep well and the dynamic loading of the pumping string and other apparatus is less.
Rotating drum or winch type long stroke pumping units have come into use in recent years. In these devices a flexible linear power transmitting member extends from the winch or capstan of the pumping apparatus to the pumping string and a second flexible linear power transmitting member, for example, a wire cable, extends from the capstan drum to a counterweight which at least partially balances the weight of the pumping string. As the capstan rotates first in one direction and then in the other, the linear members are alternately wound upon the capstan drum and unwound and the reciprocal movement thus imparted to the linear members is transferred to the pumping string and the counterweight. Pumping units such as described are shown, by way of example in U.S. Pat. Nos. 3,285,081 to Kuhns et al. and 3,640,324 to the present inventor.
Pumping units such as shown in U.S. Pat. No. 3,285,081 to Kuhns et al. depend upon changing dynamic relationships between the relative effective weight of the pumping string and oil in the well tubing and the counterweight to turn the pumping stroke around at the ends of the stroke. The motor driving the apparatus is operated only during the central portion of the pumping stroke, the energization and deenergization of the motor being controlled by limit switches or other control means responsive to the operative position of the pumping string or capstan with respect to the pumping stroke. Such long stroke pumping units permit a very long pumping stroke with low torque and reduce the number of strokes and thus fatigue cycles to lift a given amount of well fluid. Such pumping units also have the advantage of minimum acceleration of the rod and fluid loads.
These long stroke pumping units have still, however, been subject to detrimental oscillations or harmonics in the pumping string, which oscillations interfere with efficient pumping and seriously increase the loading and fatigue in the pumping string, the components of the downhole pump and the above ground pumping apparatus.